1. Field of the Invention
This invention relates generally to structural members, and more particularly to improved solid or hollow core structural members formed of a cement-based slurry infiltrated fiber composite material wherein the walls of the structural member contains a mass of short fibers or fiber mats of organic or inorganic materials having a predetermined fiber volume density completely infiltrated in a cement-based matrix mixture.
2. Brief Description of the Prior Art
Structural beams, columns, and poles, such as; railroad ties for supporting steel railroad tracks, telephone and utility poles, bridge or highway overpass support beams and columns, pilings for building foundations and piers, and culverts, are usually constructed of wood, reinforced concrete, pre-stressed concrete, metal, or fiberglass.
Environmental enclosure structures such as: secondary containment vaults for hazardous materials; underground storage vaults; controlled environment vaults for housing communication security vaults for storing volatile explosives, nuclear weapons, test devices, weapons components, and radioactive wastes, are usually fabricated using conventional reinforced and pre-stressed concrete.
Since the early 1800's, railroad ties and telegraph poles have been made from wood. In the early 1970's, the precast pre-stressed concrete tie was introduced commercially by Costain Concrete Tie Company in Alberta, Canada, and shortly thereafter in the United States. In 1986, the company relocated to Spokane, Wash. and changed their name to CXT, Inc. Concrete ties have been gaining popularity among railroad companies since their inception in 1971. Also in the early 1970's, concrete telephone and utility poles were commercialized by Centrecon (American Pole Products) of Evererett, Wash.
Demand for wood-alternative structural members has steadily increased due to rising timber prices and environmental restructions. With the recent (1993) cut-back in federal timber sales in the Northwest, the use of wood as a structural material will drop substantially in the nineties. Timber prices are projected to rise 35% in 1993 and an additional 10% in 1994.
Conventional structural members formed of wood or concrete are subject to cracking and deterioration because of environmental changes, such as freeze-thaw and/or moisture-heat cycles. These same conditions cause steel structural members to rust or corrode. In the case of telephone and utility poles, wind conditions will adversely affect the wood, concrete or steel structures because they are subjected to vibration and bending movements (and earthquakes in some areas) which cause cracking, spalling, and deterioration. In the case of railroad ties, abrasion occurs on the bottom side of the ties due to particles of hard material, such as locomotive traction sand, which is 10 times the hardness of hydrated cement. This condition prematurely wears out the concrete ties.
Also, concrete railroad ties and most other concrete structures have an alkali-aggregate reaction which results from certain types of silica in the aggregate reacting with alkalis in the cement to form a gel. The gel absorbs water,from the air or ground and swells, thus causing severe crazing, followed by expansion of the concrete and severe cracking. In pre-stressed concrete structures, the result is loss of bond and, hence, pre-stress, which leads to structural failure.
Earthquakes can cause failure of pre-stressed concrete support beams and columns. For example, in the San Francisco freeway disaster in 1990 the bridge and highway overpass support beams and columns collapsed due to very low flexural properties, and resulted in the loss of life and millions of dollars in damages.
In the past, materials such as petroleum products, chemicals, and hazardous materials have been stored in large metal or fiberglass tanks which are buried underground. Most of these "underground fuel storage tanks" (UFST) are prone to leakage due to being subjected to the hydrostatic forces of ground water, physical stresses associated with ground movement, and the corrosive action of soil environments. Great damage to the environment and personal injury often results when the leaked materials enter the soil or ground water. The United States Environmental Protection Agency. (EPA) has recently adopted regulations for Underground Fuel Storage Tanks (UFST) in response to the growing awareness of the damage caused by releases from the UFST's.
One method to comply with the EPA regulations is to place the fuel storage tank inside a buried "secondary containment vault" which allows the tank to be monitored for leakage and, in the event of a leak, will contain the leak to prevent the material from entering the soil or ground water. The secondary containment vault also isolates the fuel tank from soil and hydrostatic pressures and the corrosive action of many soils. Most underground secondary containment vaults currently available are fabricated using conventional reinforced and pre-stressed concrete. To meet the structural design requirements for resisting hydrostatic loads and soil pressures, the walls of the vaults are generally from 8 to 10 inches thick.
Other structures, such as controlled environment vaults and high security vaults are usually fabricated using conventional reinforced and pre-stressed concrete. The controlled environment vault is a box-like structure used for housing communication equipment, such as telephone, computer, or surveillance equipment, etc., and may contain temperature control equipment, dehumidifiers, fresh air blowers, environment monitors and alarms, and electrical control panels and outlets, etc. to provide a controlled environment. The controlled environment vaults may be partially buried with an entry hatch above ground. Controlled environment vaults range in size from about 17'-25' in length, 7'-12' in height, and 10'-12' in width. A controlled environment vault of conventional steel reinforced concrete in the smaller size has a weight of 70,000 lbs, and the larger size weighs about 140,000 lbs, with a concrete strength of 5,000 psi. The high security vault is a box-like structure used for storing volatile explosives, nuclear weapons, test devices, weapons components, and radioactive wastes, where high strength and security is a factor.
Utility Vault Company, Inc., of Chandler, Ariz. manufactures secondary containment vaults, and controlled environment vaults which are constructed of conventional steel reinforced concrete.
There are several patents which disclose various fiber reinforced concrete structures.
U.S. Pat. No. 3,429,094 to Romualdi discloses a two-phase concrete and steel material comprising closely spaced short wire segments uniformly distributed randomly in concrete wherein the average spacing between wire segments is not greater than 0.5 inches.
Fleischer et al, U.S. Pat. No. 4,257,912 discloses a system for fixed storage of spent nuclear fuel having activated fission products contained within a metallic fuel rod housing which comprises a uniform concrete contiguously and completely surrounding the metallic housing which has metallic fibers to enhance thermal conductivity and polymers to enhance impermeability for convectively cooling the exterior surface of the concrete.
Rotondo et al, U.S. Pat. No. 4,404,786 discloses a method and apparatus for making reinforced concrete products including hollow poles wherein arrays of reinforcing rods are distributed and embedded automatically during the introduction of concrete into a form.
Lankard et al, U.S. Pat. No. 4,559,881 discloses a burglar resistant security vault formed of prefabricated steel fiber reinforced concrete modular panels wherein Portland Cement, fly ash, fine aggregate, gravel and water are mixed for an extraordinarily long period of time and they remain a mass of crumbly, damp, powder and aggregate until the superplasticizer admixture is added, at which time the mixture reaches a fluid state. Steel fibers are then added to the mixture and mixing continues until the mixture including the steel fibers is poured into a mold cavity.
Double et al, U.S. Pat. No. 4,780,141 discloses a cementious composite material containing metal fiber which particularly formulated to have high strength and a high degree of vacuum integrity at high temperatures. The composite comprises a high strength cement matrix and a filler component comprising a metal fiber having a length of about 0.05 mm. to about 5 mm. (about 0.02" to about 0.20"). The metal fiber filler is mixed with the cement matrix at a high vacuum to minimize air bubbles and then the liquid mixture (including metal fiber) is poured into the mold.
Heintzelman et al, U.S. Pat. No. 5,030,033 discloses a conventional concrete underground storage vault comprised of a plurality of concrete sections sealingly secured together with grout keys and joint wrap. A fluid and material resistant (epoxy) coating is applied to the interior surfaces and an inert gas atmosphere is maintained within the vault to inhibit influx of oxygen and moisture. There is no teaching in Heintzelman of the type of concrete used, other than "precast concrete" or "steel and/or concrete".
Riley et al, U.S. Pat. No. 4,133,928 discloses a composite cementious or gypsum matrix material having precombined absorbent fibres and reinforcing fibre embedded therein. The absorbent fibres are selected from the group consisting of cotton, wool, cellulose, viscose rayon, and cuprammonium rayon, with the reinforcing fibers being selected from the group consisting of glass, steel, carbon, polyethylene and polypropylene. The fibre combinations are impregnated with portland cement or gypsum. Riley et al teaches a steel wire/cotton yarn reinforced concrete made by loom weaving a tape or felt having ten ends per inch for each fibre in both the longitudinal (warp) and cross (weft) directions then passing the tapes through a portland cement mortar slurry consisting of one part water, two parts cement, three parts sand by weight, and then winding the tapes into a mold and placing the mold in a curing room for one month.
As described hereinafter, the present invention utilizes a "cement-based slurry infiltrated fiber composite" construction which is significantly different from conventional "steel bar reinforced concrete", "steel fiber reinforced concrete", and "pre-stressed concrete", in both its fiber volume density and in the manner in which it is made. The "cement-based slurry infiltrated fiber composite" described hereinafter overcomes the disadvantages of conventional concrete structural members and produces a structure which has thinner walls and a gross weight significantly less than conventional reinforced and pre-stressed concrete structures of the same size and has the same or greater strength characteristics, and a much higher bending capacity approximating that of structural steel
The present invention is distinguished over the prior art in general, and these patents in ' particular by improved structural members including solid and hollow core beams, poles, columns and enclosure structures which are formed of a cement-based slurry infiltrated fiber composite material. The improved structural members are produced by first placing a plurality of individual short fibers or fiber mats of organic or inorganic materials into a form to create a bed of fibers substantially filling the form and having a predetermined fiber volume density and then adding a cement-based slurry mixture into the form to completely infiltrate the spaces between the fibers. Existing structural members may be retrofitted with jackets of the cement-based slurry infiltrated fiber composite material. The cement-based slurry mixture includes a composition of Portland cement or blended cement, fly ash, water, a high-range water reducer (superplasticizer), and may also include fine grain sand, ground granulated blast-furnace slag, chemical admixtures, and other additives. Due to its fiber volume density, and method of manufacture, the resulting structure has greater strength, less maintenance, and less cracking and deterioration than wood, steel, or conventional reinforced concrete and pre-stressed concrete structures, and a much higher bending capacity approximating that of structural steel.